The invention relates generally to stress-strain measurements and more particularly to resistance type strain gages.
In nuclear testing underground gages are needed to obtain measurements that can provide components of the stress response during shock loading in the underground geologic material. Such measurements provide an empirical basis for phenomenological concepts of containment and provide data to validate predictive analysis of the stress history during shock loading. A gage is desired which measures three diagonal components of the elastic stress or strain tensor in a body or formation. Applications of interest include shock wave propagation geometries where multi-components of the stress-strain tensors are to be measured. The value of stress and strain measurements for multi-components of the stress-strain tensors is significant to materiai testing and va1idation of material models and computer codes. Use will be extensive in shock loadings.
Strain gages are used to measure mechanical deformation. Resistance (piezoresistance) type strain gages operate on the basis of measuring the change in electrical resistance of a metal wire or foil that results from the change in length caused by the applied stress or strain. A typical gage is constructed of a very fine wire (e.g. 0.001" diameter) or foil of a high resistance metal on a backing material which is cemented to the surface of the structural element. To obtain a higher resistance, the conductor is folded in a zig-zag pattern. Gage resistances are typically 50-5000 ohms and gage sizes from a fraction to several inches. The strain gage is often used in a bridge circuit, e.g. a Wheatstone bridge. As the resistance of the gage changes in response to the stress, the output voltage of the bridge changes, and is measured to determine stress or strain.
However, current gage designs using stress-strain sensitive resistivity material such as ytterbium or manganin are limited to only one response measurement of resistance changes during loading or deformation history. Gages are often surface mounted and provide a measurement of only one-dimensional stress. In a multi-component stress or strain field it is desirable to measure the diagonal components of the stress-strain tensor. However, existin gages measure only one piezoresistance signal from which only one stress component of the stress tensor can be obtained, or possibly the "pressure" in some particular situations. For shock wave applications, the one component piezoresistivity gage is not reliable for stress components that are normal to the direction of shock propagation. Hence it is desired to provide a strain gage for measurements of three diagonal components of the stress-strain tensor in a stress-strain field, and in particula to make measurements normal to a radially propagating shock front.
Illustrative prior art gages include the following:
U.S. Pat. No. 3,453,873 to Lambert discloses a surface mounted strain gage having a plurality of elements located in the quadrants of a measurement plane.
U.S. Pat. No. 3,543,568 to Russel shows a surface mounted vertically stacked strain gage assembly, a11 of the same material, to produce a higher gain (amplify the output).
U.S. Pat. No. 4,185,496 to Tisone shows a thin film strain gage deposited on the surface of a flexure beam.
U.S. Pat. No. 4,546,652 to Virkar is not a strain gage but a detector having a plurality of conducting strips mounted to the surface of a structure with means for detecting circuit discontinuities caused by cracks in the structure.